Allow Rails/ReflectionClassName to use symbol argument

[unasuke]

belongs_to :account, class_name: :Account

Rails/ReflectionClassName ( introduced from #6704 ) offences that code, but symbol is also uses as string so I think this code has no offences by the cop.

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Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[koic]

That makes sense. Can you write the CHANGELOG entry?

[unasuke]

@koic I see!

[koic]

Unfortunately I had overlooked the discussion below 💦
#6704 (comment)

I have an opportunity to meet Rails committer soon. I'd like to make a final judgment by listening to opinion as Rails. Please wait for a little while longer.

[Drenmi]

Looks like a number of different test cases have been squashed into one (even before you made this change.) Could you please separate the test cases so we test one thing per test? 🙂

[koic]

Correct 💦 The test case of String object and the test case of Symbol object should have been independent.

[mikegee]

I disagree with allowing a symbol here. It is immediately converted to a string with to_s, so we are ok requiring a string in the first place.

[koic]

I think that using String for class_name will be more unified than using Symbol. However I'd like to accept this change. Because the following test case that accepts Symbol with class_name is written in test cases of Rails.
https://github.com/rails/rails/blob/v6.0.0.beta2/activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb#L950-L955

On the other hand, If you propose to Rails that it does not accept Symbol with class_name and accept it, let's change it so that Rails/ReflectionClassName cop does not accept Symbol at that time.

Cc @amatsuda @kamipo @y-yagi

[koic]

Thanks!

[unasuke]

Thank you too!